Thermocolor ribbons have long been known. They generally comprise a foil-like carrier or support, for example of paper, a plastic or the like, and a melt color which can be composed of a plastic-bonded and/or wax-bonded coloring agent or carbon black.
The melt color of the thermocolor ribbons is brought into a molten state by a thermal printing head which heats the melt color and causes transfer of a molten portion locally to a substrate which can be a printer paper.
Thermal printers and thermal printer heads which can be used for this purpose are described, for example, in the German Patent Documents DE-AS 20 62 494 and DE-AS 24 06 613 as well as De-OS 32 24 445.
In the thermal printing head of a printer, heated points or pins can define a symbol to be printed, e.g. an alphanumeric symbol which is to be applied in the form of a corresponding pattern of the melt color to a paper sheet. The thermal printing head presses the thermocolor ribbon against the paper to be imprinted. The heated alphanumeric symbol of the thermal printing head applies a temperature of about 400.degree. C. to the thermocolor ribbon to locally melt the color transfer layer at the heated locations and cause transfer to the paper sheet at these locations at which the paper sheet contacts the ribbon.
The used part of the thermocolor ribbon is taken up on a spool.
The thermocolor ribbon can have a variety of melt colors next to one another. With the combination of the basic colors blue, yellow and red, for example, colored images can be printed. This system has a significant advantage over conventional color photography since the development and fixing steps required for such photography can be eliminated.
Thermal printers can operate with high writing speeds, for example, to print a German Industrial Standard DIN A4 page in about 10 seconds, without significant noise generation.
Apart from this kind of thermal ribbon, there is also a thermal ribbon which does not require the formation of a heated symbol on a thermal printing head to effect the transfer of the melt color to the substrate.
With such a ribbon, the melt color is heated by resistance heating generated by a specially formed foil-like carrier. The heat is generated by passing an electric current through the latter and the melt color and/or the carrier must contain electrically conductive materials. Apart from such electrically conductive materials, however, the melt color has the composition previously described. In the field, this type of ribbon is referred to as an ETR material, namely, an electrothermal ribbon. A Thermal transfer printing system using this ribbon is described, for example, in U.S. Pat. No. 4,309,117.
It has been found that many thermocarbon ribbons of the above-described type have unsatisfactory covering power during the printing process. In other words, the optical density of the image produced on the substrate, i.e. the receiving paper sheet, leaves much to be desired.